This invention is in the field of heating and cooling devices and methods of operating the same.
Measurements of air conditioners have shown that in the southern United States some 40% of refrigerant capacity is expended in condensing water vapor. Not more than one-third of this is necessary for cooling or for comfort; the rest is wasted. While most of the condensed water drains outside of the building, there is always a quart or so left adhering to the cooling plates when the compressor is turned off by the thermostat. This can be evaporated by allowing the blower to continue for some 15 minutes after the compressor is turned off, thus providing additional cooling to the air stream. Since the amount of cold recovered is roughly the same every time the compressor is turned off, it is advantageous to turn it off as often as possible. However, reducing the time that the compressor is on, while fixing the time the blower continues after the compressor is turned off, reduces the net cooling capacity of the system. When the compressor comes on intermittently, this is of no consequence; but at peak load (7 to 8 pm), the thermostat may well require that the compressor is in continuous operation. At the time of peak load, and with no restriction on the time for which the compressor is turned on, then, there is no evaporation and the condensed water is lost. With the Efficiency Control, recovery is effected by interrupting the compressor, and fixing an upper limit to the time it is allowed to operate. A compromise which gives good recovery (on average two-thirds of that 40%) and adequate capacity is obtained by making this upper limit also 15 minutes. This constraint is the more important because thermostats can be sluggish in response, unfortunately located in pockets of stagnant air; or, owing to poor insulation, construction, or design of houses, air conditioners are, more often than not, of insufficient capacity to cope with the peak load. It follows that for a few hours near the time of the peak load, the temperature within the house will rise a degree or two above that set by the thermostat, a rise of which most residents are unaware. With the Efficiency Control in operation, this temperature rise will be a little higher, but again to no noticeable degree, and at less power consumption by a third.
A better understanding may be obtained by considering the cooling process in more detail. As the compressor is turned on, the blower being in continuous operation, there is a rapid and nearly exponential fall in temperature of the air stream as cold is stored in the cooler and ducts. When the compressor is turned off, there is a similar rapid rise in temperature, followed by a long tail, usually lasting 20 minutes. All air conditioners show this tail, roughly in the same proportion and lasting for the same time. It is caused by the evaporation of water (2 to 4 pints) which adhere to the 100 square feet or so of cooling plates. It is the cold represented by this tail, resulting from the evaporation of condensed water, which is passed into the air stream and prolongs the cooling of the air conditioner. A roughly equal cooling effect is, therefore, restored to the house every time the air conditioner is turned off provided that the blower is allowed to continue after the turning off of the compressor. Reducing the time that the compressor is on has the effect of requiring it to come on more often with increasing contributions of cold passed on to the house. If this time is too short, no water is condensed and the temperature within the house would fall to the dew point, where the humidity is 100 percent. Although conditions commonly occurring would make this very unlikely, too short a time certainly leads to excessive humidity. At the time of peak load, and with (as is usually the case) an inadequate air conditioner, the compressor will come on for the maximum time allowed by the Efficiency Control. The longer this time, the more water is lost to the system by condensation and less cold is recoverable. By making it 15 minutes, about two-thirds of the condensed water is evaporated in the 15 minutes that the blower is on following the turning off of the compressor. This is the basis of the compromise referred to above. In these conditions, then, the same cooling rate is provided with the Efficiency Control in operation for about two-thirds of the electric power, at the price, however, of an average cooling capacity less than without it, and an increase in humidity of a few percent. Taking into account times other than that of peak load, the overall improvement in power consumption is even better.
Centrally cooled and heated systems are conventionally controlled by thermostats fitted with an `auto` position, the blower coming on automatically when the compressor is turned on, or with an `on` position where the blower is on continuously until turned off manually. Without the Efficiency Control, the `auto` position has nothing to recommend it, for no condensed water is evaporated, and serious losses can occur when the compressor and blower are not running. In the `on` position, power savings are obtained because the same process of evaporation described above can occur when the thermostat interrupts the compressor. However, such savings are lower than those obtained with the Efficiency Control in operation, because there is no interruption of the compressor at times of peak load, and the blower, being on continuously, runs longer than is necessary.
Although furnace operation has nothing equivalent to the cold recoverable from evaporation of condensed water in air conditioners, measurements show that there is always more heat recoverable from a furnace cooling down after being turned off, than is put into it when heating up, the difference being independent of the time for which the furnace is turned on. The explanation of this surprising fact lies in the two-stage process which determines heat transfer in a furnace. As with air conditioners, then, it is advantageous to turn the furnace on and off as often as possible. Measurements show that a suitable maximum furnace `on` time is 10 minutes, while limiting the blower to run for 10 minutes after the furnace is turned off. The savings of fuel which result depend on circumstances, but will usually amount to some 20 percent.
Blower controls for removal of stored heat have been suggested in U.S. Pat. Nos. granted to J. F. Page, 2,835,448, G. E. Elwart, 3,454,078, and C. D. Moreland, 3,489,345, of which the most noticeable common feature is the proposed variation of the speed of the blower, slower speeds at appropriate times supposedly increasing comfort. This may be so, but there can be no doubt that decreasing the flow of air can only decrease the efficiency of a furnace, whatever its temperature, if only because the heat transfer between furnace walls and air stream depends on the Reynolds number of the latter: decreasing the air velocity produces a nearly proportional decrease in heat transfer, with a corresponding increase in temperature of the furnace walls. Since the primary transfer of heat between the furnace flames and these walls is almost entirely radiative, increasing the wall temperature sharply reduces the heat transfer from within, with, consequently, more heat going up the chimney. Other U.S. Pat. Nos. granted, e. g. those to D. N. Joslin, 3,599,710, S. Sapir, 3,726,473, and F. T. Bauer, 3,912,162, are similar. All are concerned with improvements in comfort; none involve any real improvement in fuel consumption, and where suggested that such improvements might result, are quite clearly in error. The prior patents referred to depend on the temperature of the air being treated for their operation.